The present invention relates to a non-contacting seal for minimizing leakage between fixed and rotatable elements and particularly relates to a two-directional permanent magnetic seal system that floats, i.e., levitates, a seal ring about a rotating shaft with small radial clearance between the shaft and seal ring and small axial clearances between opposite faces of the seal ring and a housing whereby fluid leakage through the seal may be controlled.
Many and various types of sealing systems have been used for controlling leakage in rotating equipment, such as steam turbines. Labyrinth seals, for example, used in steam turbines are non-contacting and, therefore, have long life. Their relatively high leakage rates, however, require an auxiliary gland seal system. Such gland seal systems include highly complex and heavy seal regulators for reducing high pressure steam to the few pounds per square inch required for gland sealing. Several manual valves, high and low pressure drains, and several lengths of steam piping constitute additional complexities for such systems. Moreover, gland exhaust systems also require air ejectors, a gland exhaust condenser and piping to carry the steam and air away from the system. Additionally, weight and space requirements for these systems are high and hence costly, for example, where steam turbines are used in a ship's engine room.
Carbon ring seals are contacting seals that also minimize leakage, e.g., steam leakage. Carbon ring seals, however, have relatively high wear rates.
Rotary gap type seals are also known, such as shown in U.S. Pat. No. 3,499,653. In that patent, gas seals are disclosed which are non-contacting at high speed and control leakage by using hydrodynamic forces to control separation between sealing faces. At low speeds and/or pressures, however, some contact usually occurs resulting in wear and possible damage.
From U.S. Pat. Nos. 4,434,987 and 4,447,063, it is known to use an electromagnet to control a gap between a rotating ring and a surrounding non-rotating ring in shaft seals where sealing fluid is pumped into the gap and exhausted through a bore in the stator elements. These are positive systems employing multiple parts and hence are costly and complex.
The permanent magnet floating shaft seal of the present invention is designed to overcome the limitations of the conventional sealing systems discussed above, i.e., to minimize fluid leakage, maximize life and wear, reduce weight, complexity, volume and costs. According to the present invention, an annular radially extending gap between a floating seal ring and a rotating shaft, as well as axially extending gaps between the seal ring and shaft housing, are magnetically and therefore passively controlled by directional permanent magnets to maintain those gaps substantially constant with minimum clearance, hence controlling fluid leakage. The seal ring levitates axially and radially to compensate for thermal expansion, vibrations and the like that tend to open or close the gaps between the sealing faces. Because of the non-contacting nature of the seal ring, vis-a-vis, the housing and rotating shaft, the various sealing parts do not wear. Further, the seal ring is free to move radially to follow radial and other vibratory shaft excursions, as well as permit relative axial movement between the seal ring and shaft.
To accomplish the foregoing, the permanent magnetic floating shaft seal of the present invention includes three distinct parts, each having a permanent magnet cooperable with a permanent magnet of at least one adjoining part. The three parts include a seal ring having two separate and distinct directional permanent magnets, a housing having a directional permanent magnet, and a shaft having a directional permanent magnet. More particularly, the shaft has a first annular permanent magnet extending annularly about its outer surface. The seal ring includes a second annular magnet in spaced radial opposition to the first permanent magnet of the shaft. The poles of the permanent magnets are arranged such that magnetic lines of force serve to repel the shaft and seal ring away from one another whereby the seal ring levitates in a spaced radial direction relative to the shaft with the radial gap therebetween being maintained substantially constant. The seal ring includes third and fourth annular directional permanent magnets disposed along the respective opposite axial faces thereof. Fifth and sixth annular directional permanent magnets are disposed in axial opposition to the third and fourth permanent magnets of the seal ring, respectively, such that the opposing magnets, i.e., the third and fifth permanent magnets and the fourth and sixth permanent magnets, provide respective axially extending lines of force which repel one another. Accordingly, the seal ring is not only levitated in a radial direction but axially between the permanent magnets of the housing. The seal ring, therefore, lies out of contact with the housing and shaft with the gaps formed between the seal ring and shaft and the seal and ring housing, providing extremely small clearances affording minimum leakage. Thus, the seal ring is free to move radially to follow radial excursions of the shaft. The shaft is also free to move axially relative to the seal ring. The shaft is free to move axially because the axial movement is at right angles to the levitation forces (flux lines). The resisting forces at right angles to the flux lines are small in comparison to the directional magnetic levitation forces.
To ensure levitation of the seal ring in a radial direction, notwithstanding axial movement of the shaft relative to the seal ring, the radially opposed directional permanent magnets of the seal ring and shaft have different axial lengths. Thus, when the shaft has the axially wider magnet, for example, the movement of the shaft relative to the seal ring in the axial direction causes no change in the magnetic forces exerted between the two parts affording levitation of the seal ring in the radial direction. Conversely, the magnet on the seal ring may be wider than the opposing magnet on the shaft without adverse effect on the magnitude of the magnetic forces therebetween.
In a preferred embodiment according to the present invention, there is provided a seal for minimizing leakage between fixed and rotatable elements comprising a shaft mounted for rotation about an axis, the shaft having a first permanent magnet about the shaft, a housing disposed about and spaced from the shaft, a seal ring in the housing about the shaft, the seal ring being spaced from the shaft and defining a circumferentially extending radial gap therewith, the seal ring carrying a second permanent magnet at a like axial position about the shaft as the first permanent magnet and in radially spaced opposition thereto, with means for maintaining the seal ring spaced from the housing with a minimum clearance between the housing and the seal ring, the first and second permanent magnets generating repelling forces therebetween to enable the seal ring to maintain a substantially constant annular gap between the seal ring and the shaft at each circumferential position thereabout to minimize leakage between the seal ring and shaft.
In a further preferred embodiment according to the present invention, there is provided a seal for minimizing leakage between fixed and rotatable elements comprising a shaft mounted for rotation about an axis, a housing disposed about and spaced from the shaft, a seal ring in the housing about the shaft, the seal ring being spaced from the shaft and defining a circumferentially extending radial gap therebetween, a pair of annular permanent magnets carried by the shaft and the seal ring at like axial positions and in radially spaced opposition to one another, a second pair of permanent magnets carried by the seal ring and the housing, respectively, in axial opposition to one another on one side of the seal ring to define an axially extending gap between the seal ring and the housing, and a third pair of permanent magnets carried by the seal ring and the housing, respectively, in axial opposition to one another on the opposite side of the seal ring to define a second axially extending gap between the seal ring and the housing, the pairs of permanent magnets generating repelling forces therebetween to enable the seal ring to maintain substantially constant gaps between it and the shaft and housing to minimize leakage past the seal.
Accordingly, it is a principal object of the present invention to provide a novel and improved seal between fixed and rotatable elements which minimizes fluid leakage therebetween, maximizes the life of the parts of the seal and reduces the weight, volume and cost of the seal.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.